1. Field of the Invention
The present invention relates to an apparatus for controlling charging of a storage battery.
2. Description of the Prior Art
Conventionally, an apparatus for controlling charging of a storage battery has been provided for supplying power from a power supply to the storage battery to perform rapid charging with a constant current and then stopping the current to the storage battery at completion.
FIG. 1 shows a circuit diagram of a conventional apparatus for controlling the charging of the storage battery. The illustrated apparatus comprises a power supply 1 for supplying a predetermined current, a storage battery 2 for receiving the current from the power supply 1 through a diode D1, a charged voltage state detector 3 including a zener diode ZD1, resistors R1-R4, and a capacitor C1 for checking the charging state of the storage battery 2 to output first and second voltage signals V1 and V2 in accordance with the result, a peak value detector 4 including resistors R5 and R6, an operational amplifier OP1, a diode D2, and a capacitor C4 for receiving the second voltage signal V2 from the charged voltage state detector 3 to detect a peak value of the second voltage signal V2, a peak value holder 5 including resistors R7-R10, capacitors C2 and C3, a transistor Q1, and a diode D3 for constantly holding an output signal V3 from the peak value detector 4, and a comparator 6 including an operational amplifier OP2 and a capacitor C5 for receiving the first voltage signal V1 from the charged voltage state detector 3 and the output signal V3 from the peak value detector 4 and comparing the received signals V1 and V3 to produce an output signal V4 as a power-on signal or a power-off signal to the power supply 1 in accordance with the compared result.
The operation of the conventional apparatus with the above mentioned construction will now be described.
First, upon being turned on, the power supply 1 supplies a constant current to the storage battery 2 through diode D1, so that the storage battery 2 is rapidly charged with the constant current. During the charging of the storage battery 2, a small current flows through the charged voltage state detector 3. Namely, the small current passes through resistors R1, R2, and R3 in that order and then flows through capacitor C1 and resistor R4. As a result, the first and second voltage signals V1 and V2 are output from the charged voltage state detector 3.
Note that during the charging of the storage battery 2, the first voltage signal V1 has a value higher than that of the second voltage signal V2.
The first voltage signal V1 is applied to the non-inverting input terminal (+) of the operational amplifier OP2 in the comparator 6 and the second voltage signal V2 is applied to the non-inverting input terminal (+) of the operational amplifier OP1 in the peak value detector 4 through resistor R5.
The peak value is always detected at capacitor C4 in the peak value detector 4 and the detected peak value is then applied to the inverting input terminal (-) of the operational amplifier OP2 in the comparator 6. For this reason, during the charging of the storage battery 2, the comparator 6 produces a "high" state output signal V4. This high output signal V4 is applied to the power supply 1 so that the power supply 1 remains on for the continuous charging of the storage battery 2.
In the peak value holder 5, transistor Q1 is turned on in response to an external input signal V5 during the charging of the storage battery 2, thereby causing the voltage across capacitor C4 in the peak value detector 4 to be discharged. As a result, the peak value holder 5 holds the output signal from the peak value detector 4 constant.
If the charging of the storage battery 2 is completed at instant t0, for example, in FIG. 2A, a large amount of current flows through resistor R1 in the charged voltage state detector 3. Also, if the amount of the flowing current exceeds a predetermined value, the zener diode ZD1 becomes shorted. For this reason, the current flowing through resistor R1 flows through the zener diode ZD1, not through resistors R2 and R3, and then flows through capacitor C1 and resistor R4. At this time, the second voltage signal V2 has a value higher than that of the first voltage signal V1.
As a result, the output signal V3 from the peak value detector 4 is higher than the first voltage signal V1 from the charged voltage state detector 3 so that the comparator 6 outputs a "low" state output signal V4. This low output signal V4 is applied to the power supply 1 to turn off the power supply 1.
With reference to FIG. 2A, a more detailed description of the above operation of the conventional apparatus will be presented.
During the time interval 0-t0 in which the charging of the storage battery 2 is advanced, the first voltage signal V1 from the charged voltage state detector 3 is higher than the output signal V3 from the peak value detector 4. Thus, comparator 6 outputs a high signal V4, thereby allowing the power supply 1 to remain turned on. The voltage across the storage battery 2 is maximum at the instant t0 and, at instant t1 after the lapse of a predetermined period of time T, the first voltage signal V1 from the charged voltage state detector 3 is lower than the output signal V3 from the peak value detector 4. Thus, comparator 6 outputs a low signal V4.
The conventional apparatus has the following disadvantages.
First, as shown in FIG. 2b, since the charging operation begins at the instant t0 in which the voltage charged into the storage battery 2 is maximum and is completed at the instant t1 in which the charged voltage is decreased by voltage variation .DELTA.V to a desired value of the charged voltage, an overcurrent is needed. The overcurrent is supplied to the storage battery 2 during a detection period of time T during the variation .DELTA.V, thereby causing the life of the storage battery 2 to be shortened.
Second, since the maximum value of the charging current necessary for the rapid charging is decreased for the protection of the storage battery 2 from the overcurrent, the charging period is lengthened.
Third, a costly timer must be added separately to protect against an unexpected variation of the voltage of the storage battery 2 or a malfunction of a temperature detecting circuit.